* fix: fix RelativeTimeFormat type definition
Changes:
1. Change BCP47LanguageTag to UnicodeBCP47LocaleIdentifier: Those mean 2
different things. BCP47LangTag allows _ as separator while UTS35
doesn't. It also allows grandfathered locales and UTS35 doesn't.
2. Combine RelativeTimeFormat interface and const declaration into a
single class. The old way of declaring as `interface` & `const` permits
calling `Intl.RelativeTimeFormat` without `new` which is no longer
possible after `Intl.DateTimeFormat` & `Intl.NumberFormat`. The spec
explicitly forbids it in
http://ecma-international.org/ecma-402/7.0/index.html#relativetimeformat-objects
where:
> If NewTarget is undefined, throw a TypeError exception.
Intl.RelativeTimeFormat is also extensible per spec. This is closer to a
`class` than the current declaration.
* address feedbacks
Long Hoandlong.ho